1. Field of the Invention
The invention relates to an apparatus for forming an image by means of radiation, including a radiation detector unit for picking up the image, which radiation detector unit includes image detector elements which are arranged in a matrix of rows and columns and each of which includes an image sensor element, being sensitive to the imaging radiation, and a controllable switching element which is connected thereto, each column being subdivided into a number of groups of image detector elements, a read conductor being provided for each group of image detectors, which read conductor is connectable, via the associated controllable switching element, to one image sensor element of the group of image sensor elements associated with the relevant column, read drivers for driving the switching elements so as to read an electric variable from the image detector elements, the output of each read driver being connected to an associated control conductor which is connected to a respective control electrode of a controllable switching element of a row of image detector elements.
2. Description of Related Art
A device for reading an imaging sensor matrix is known from U.S. Pat. No. 5,120,964. The known device can be used in an apparatus for forming an image by means of radiation, for example a medical X-ray tomography apparatus.
Generally speaking, apparatus for forming an image by means of radiation, such as an X-ray image for medical purposes, are provided with a radiation detector unit for picking up the image. This unit may be constructed as a matrix of radiation-sensitive elements, i.e. the image detector elements. Each of these image detector elements consists of an image sensor element which is sensitive to the imaging radiation and of a controllable switching element (the switch) which is connected thereto. Upon exposure to the imaging radiation, an electric charge proportional to the radiation dose, i.e. to the intensity of the radiation, is stored in the image sensor element. The image is then obtained by reading the individual image sensor elements; this is performed by connecting the sensor element, via the associated switching element, to a read amplifier which then outputs an electric voltage which is a measure of the radiation intensity of the relevant sensor element. The total set of voltages then represents the desired image in the form of voltage values.
For the reading of the matrix it is generally known to drive each time a complete row of image detector elements and to apply the charge thus read to a column conductor which is associated with a respective column of such elements and acts as a read conductor, said column conductor being connected to an associated read amplifier. In devices of this kind, however, a problem is encountered in that stray capacitances have an adverse effect on reading. Even when all switches are in the non-conductive state (with the exception of that whose sensor element is being read), they still have a stray capacitance, like the read conductor (the column conductor) itself. Consequently, noise occurs in the read signal and the reading operation requires a comparatively long period of time. This is particularly disadvantageous in the case of X-ray equipment in which images are formed in rapid succession, for example 60 images per second.
In order to mitigate this problem, according to the cited US Patent Specification each column is subdivided into a number of groups of image detector elements, a respective column conductor which acts as a read conductor being added to each group of image detectors. This column conductor is connected to a respective one of the group of image detector elements associated with the relevant column. Thus, a plurality of column conductors are provided for each group of image detector elements. During a read operation the switches of the sensor elements of one row are then switched to the conductive state via a control line (a row conductor) associated with the relevant row. The control lines are driven by read drivers which thus serve to drive the switching elements so as to read the charge from the sensor element. The output of each read driver is thus connected to an associated row conductor, the row conductor being connected to the control electrodes of the switches of a row of image detector elements. The sensor elements in the relevant row are thus connected to the associated column conductor which, therefore, is intended exclusively for the group which includes the sensor element activated at that instant. The column conductors associated with one column themselves are connected to a common collecting conductor via further electronic switches. The noise behavior is thus enhanced because each column conductor acting as a read conductor is now connected to a comparatively small part of the switches of the sensor elements of a column. The collecting conductor is also connected to a comparatively small number of switches, because only as many switches as there are column conductors per column are then required. The enhancement of the noise behavior of such a device is greater as one column contains more sensor elements. Because the number of sensor elements in one column may be of the order of one thousand in practical devices, the noise is substantially reduced in such cases.
The known device with a plurality of column conductors per column has a further advantage in that the read time for the individual rows of the matrix can be prolonged in proportion to the number of column conductors, so that the noise in the image is further reduced.
It is a drawback of this known device, however, that as the number of image detector elements per column is increased, and hence also the number of column conductors, the column conductors occupy an increasing amount of space. This is mainly due to the fact that these conductors cannot be arranged arbitrarily close to one another because of the stray capacitance then occurring between these conductors, so that the noise in the read amplifiers would become excessive again.